1. Field of the Invention
This invention relates to a laser beam recording system, and more particularly to a laser beam recording system using a laser beam scanning means in which a laser beam is modulated by an acousto-optic modulator and the zero-order diffraction light beam is used for scanning a recording material two-dimensionally.
2. Description of the Prior Art
As a recording system for recording information on microfilm, there has been known a computer output microfilmer (COM) in which the output information of a computer is recorded on a microfilm together with fixed information such as frame lines given by a form slide. The conventional COM device has generally been of the type in which the output data from the computer is once displayed on a CRT and the light image on the CRT is focused on a photosensitive microfilm through a focusing lens.
On the other hand, accompanying the progress of the laser beam scanning system providing a stable and economical laser beam recording device, there has been put into practical use a laser COM device in which a laser beam is modulated by an acousto-optic modulator and caused to two-dimensionally scan a microfilm. Since the laser COM device uses a laser beam for recording information directly on a microfilm and the laser beam has an extremely high intensity and energy, the laser COM device is able to use a microfilm having a very low sensitivity such as a heat-mode recording material like a metal thin film.
Therefore, the laser COM device is advantageous in that the wet developing process can be eliminated, which process has been necessitated in the conventional COM device using a photosensitive microfilm of silver halide type. When using a heat-mode recording material like a metal thin film, it is desirable to use the zero-order diffraction light beam of an acousto-optic modulator because of its high peak energy.
However, the zero-order diffraction light beam obtained through an acousto-optic modulator involves a problem that the intensity thereof is not made completely zero when the output level is turned to zero. In other words, the zero-order diffraction light beam has a turned ON level "1" and a turned OFF level "0" being modulated by the acousto-optic modulator. Since the intensity of the zero-order diffraction light beam is very high, the intensity thereof cannot be made completely zero even when it is turned OFF. Thus, the attenuated zero-order light beam cannot be made completely zero.
On the other hand, in the laser COM device, the laser beam scans the recording material primarily in the horizontal direction and secondarily in the vertical direction. While the laser COM device is in the non-recording state in which the zero-order diffraction light beam is turned OFF, the zero-order laser beam horizontally scans the same position repeatedly without vertically deflected. The non-recording state referred to here means the state where the film is fed from one frame to another and the laser beam is turned OFF to stop recording information. In the non-recording state in the conventional laser COM, there is a problem that the recording material is exposed to the horizontally repeatedly scanning attenuated zero-order diffraction light beam and unnecessary recording of non-information linear pattern appears on the recording material. This unnecessary recording will hereinbelow be referred to as "fog".
In order to obtain a clear microimage on the microfilm, the fog must be eliminated. It is possible to eliminate the fog by using a laser beam which is modulated by a current type modulator and stopping the laser oscillation itself during the non-recording state, or by using a shutter in the optical path of the laser beam to completely close the optical path thereof during the non-recording state. In the former case, however, the output power of the current modulated laser beam is not stable. Further, the current modulating type laser beam scanning system does not have a long life. In the latter case, the shutter must be controlled at a very high speed. In practice, it is very difficult to operate such a shutter at such a high speed and response. The electrically controllable high speed shutter has a very sophisticated structure and considerably costly.